The process that generates the sun's energy is sustained by nuclear fusion. Replicating this process in a controlled environment would yield an inexhaustible supply of energy, a commodity which is in relatively short supply presently. For example, at the present rate of consumption, it is estimated by some that the world's supply of oil will be exhausted within 100 years or less.
There is presently underway in the United States, and various foreign countries, research efforts directed to processes for generating energy by controlled sustained nuclear fusion. It has been suggested that one way to achieve such nuclear fusion is to bombard a target fuel cell, or capsule, usually containing a liquid deuterium-tritium (DT) fuel mixture, with a laser beam. The laser beam, at sufficiently high energy, would cause a plasma envelope to form on the surface of the fuel mixture which would drive the capsule inward (imploding) to heat the fuel. Under appropriate conditions, the compressed fuel will ignite in a thermonuclear burn, causing the fusion of deuterium and tritium to produce helium and give off more energy than that expended to ignite the fuel.
The physics of laser inertial confinement fusion (ICF), as this process is called, imposes several requirements on the design and material characteristics of the fuel and capsule that comprise the target of the laser beam.
By appropriate calculations and experimental efforts, it has been determined that optimum results will be obtained when a dense liquid or solid fuel is used, and the capsule containing fuel is shaped like a ball of near perfect spherical symmetry having a hollow core. The hollow core provides a vapor region which serves as a spark plug when the fuel and vapor is compressed by the implosion caused by the laser beam.
Such a capsule is described in U.S. Pat. No. 4,693,938.
In addition to the requirements of spherical configuration, the design of one embodiment also requires that the capsule containing the fuel be porous, low in density and made with materials having a low atomic number (i.e., as near to C, or H as possible). A capsule made of low density foam is mostly empty space. If porous or open celled, it will act as a wick, soaking up fuel, thereby enabling the maximum amount of fuel to fill the available space.
When liquid fuel is charged to the foam capsule, it becomes wet. For a wetted foam capsule, the internal structure must be sufficiently strong and the cells sufficiently small and uniform that capillary forces will support a liquid column about the height of the capsule diameter under the maximum acceleration that the capsule will undergo during use.
By calculation, it has been determined that the foam cells should have diameters of no more than about 1 .mu.m and be open, that is, linked to each other by passageways, and that the density of the foam be less than about 50 mg/cm.sup.3.